Not wanting to be a "Just use XYZ" guy but maybe in case everything else fails you might try to switch to NFS (which the Synology typically offers too).
Check Mount Options: Look at /proc/mounts. If using cache=none or forcedirectio, try switching to default caching (cache=standard) to see if the issue stops.
Kernel Logs (dmesg): CIFS kernel drivers usually log page errors. Run "dmesg -T | grep...